/*
 * Copyright (c) 2017 Baidu, Inc. All Rights Reserve.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package com.baidu.fsg.uid.buffer;

import com.baidu.fsg.uid.utils.PaddedAtomicLong;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import org.springframework.util.Assert;

import java.util.concurrent.atomic.AtomicLong;

/**
 * Represents a ring buffer based on array.<br>
 * Using array could improve read element performance due to the CUP cache line. To prevent
 * the side effect of False Sharing, {@link PaddedAtomicLong} is using on 'tail' and 'cursor'<p>
 * <p>
 * A ring buffer is consisted of:
 * <li><b>slots:</b> each element of the array is a slot, which is be set with a UID
 * <li><b>flags:</b> flag array corresponding the same index with the slots, indicates whether can take or put slot
 * <li><b>tail:</b> a sequence of the max slot position to produce
 * <li><b>cursor:</b> a sequence of the min slot position to consume
 *
 * @author yutianbao
 */
public class RingBuffer {
	public static final int DEFAULT_PADDING_PERCENT = 50;
	private static final Logger LOGGER = LoggerFactory.getLogger(RingBuffer.class);
	/**
	 * Constants
	 */
	private static final int START_POINT = -1;
	private static final long CAN_PUT_FLAG = 0L;
	private static final long CAN_TAKE_FLAG = 1L;
	/**
	 * The size of RingBuffer's slots, each slot hold a UID
	 */
	private final int bufferSize;
	private final long indexMask;
	private final long[] slots;
	private final PaddedAtomicLong[] flags;

	/**
	 * Tail: last position sequence to produce
	 */
	private final AtomicLong tail = new PaddedAtomicLong(START_POINT);

	/**
	 * Cursor: current position sequence to consume
	 */
	private final AtomicLong cursor = new PaddedAtomicLong(START_POINT);

	/**
	 * Threshold for trigger padding buffer
	 */
	private final int paddingThreshold;

	/**
	 * Reject put/take buffer handle policy
	 */
	private RejectedPutBufferHandler rejectedPutHandler = this::discardPutBuffer;
	private RejectedTakeBufferHandler rejectedTakeHandler = this::exceptionRejectedTakeBuffer;

	/**
	 * Executor of padding buffer
	 */
	private BufferPaddingExecutor bufferPaddingExecutor;

	/**
	 * Constructor with buffer size, paddingFactor default as {@value #DEFAULT_PADDING_PERCENT}
	 *
	 * @param bufferSize must be positive & a power of 2
	 */
	public RingBuffer(int bufferSize) {
		this(bufferSize, DEFAULT_PADDING_PERCENT);
	}

	/**
	 * Constructor with buffer size & padding factor
	 *
	 * @param bufferSize    must be positive & a power of 2
	 * @param paddingFactor percent in (0 - 100). When the count of rest available UIDs reach the threshold, it will trigger padding buffer<br>
	 *                      Sample: paddingFactor=20, bufferSize=1000 -> threshold=1000 * 20 /100,
	 *                      padding buffer will be triggered when tail-cursor<threshold
	 */
	public RingBuffer(int bufferSize, int paddingFactor) {
		// check buffer size is positive & a power of 2; padding factor in (0, 100)
		Assert.isTrue(bufferSize > 0L, "RingBuffer size must be positive");
		Assert.isTrue(Integer.bitCount(bufferSize) == 1, "RingBuffer size must be a power of 2");
		Assert.isTrue(paddingFactor > 0 && paddingFactor < 100, "RingBuffer size must be positive");

		this.bufferSize = bufferSize;
		this.indexMask = bufferSize - 1;
		this.slots = new long[bufferSize];
		this.flags = initFlags(bufferSize);

		this.paddingThreshold = bufferSize * paddingFactor / 100;
	}

	/**
	 * Put an UID in the ring & tail moved<br>
	 * We use 'synchronized' to guarantee the UID fill in slot & publish new tail sequence as atomic operations<br>
	 *
	 * <b>Note that: </b> It is recommended to put UID in a serialize way, cause we once batch generate a series UIDs and put
	 * the one by one into the buffer, so it is unnecessary put in multi-threads
	 *
	 * @param uid
	 * @return false means that the buffer is full, apply {@link RejectedPutBufferHandler}
	 */
	public synchronized boolean put(long uid) {
		long currentTail = tail.get();
		long currentCursor = cursor.get();

		// tail catches the cursor, means that you can't put any cause of RingBuffer is full
		long distance = currentTail - (currentCursor == START_POINT ? 0 : currentCursor);
		if (distance == bufferSize - 1) {
			rejectedPutHandler.rejectPutBuffer(this, uid);
			return false;
		}

		// 1. pre-check whether the flag is CAN_PUT_FLAG
		int nextTailIndex = calSlotIndex(currentTail + 1);
		if (flags[nextTailIndex].get() != CAN_PUT_FLAG) {
			rejectedPutHandler.rejectPutBuffer(this, uid);
			return false;
		}

		// 2. put UID in the next slot
		// 3. update next slot' flag to CAN_TAKE_FLAG
		// 4. publish tail with sequence increase by one
		slots[nextTailIndex] = uid;
		flags[nextTailIndex].set(CAN_TAKE_FLAG);
		tail.incrementAndGet();

		// The atomicity of operations above, guarantees by 'synchronized'. In another word,
		// the take operation can't consume the UID we just put, until the tail is published(tail.incrementAndGet())
		return true;
	}

	/**
	 * Take an UID of the ring at the next cursor, this is a lock free operation by using atomic cursor<p>
	 * <p>
	 * Before getting the UID, we also check whether reach the padding threshold,
	 * the padding buffer operation will be triggered in another thread<br>
	 * If there is no more available UID to be taken, the specified {@link RejectedTakeBufferHandler} will be applied<br>
	 *
	 * @return UID
	 * @throws IllegalStateException if the cursor moved back
	 */
	public long take() {
		// spin get next available cursor
		long currentCursor = cursor.get();
		long nextCursor = cursor.updateAndGet(old -> old == tail.get() ? old : old + 1);

		// check for safety consideration, it never occurs
		Assert.isTrue(nextCursor >= currentCursor, "Curosr can't move back");

		// trigger padding in an async-mode if reach the threshold
		long currentTail = tail.get();
		if (currentTail - nextCursor < paddingThreshold) {
			LOGGER.info("Reach the padding threshold:{}. tail:{}, cursor:{}, rest:{}", paddingThreshold, currentTail,
				nextCursor, currentTail - nextCursor);
			bufferPaddingExecutor.asyncPadding();
		}

		// cursor catch the tail, means that there is no more available UID to take
		if (nextCursor == currentCursor) {
			rejectedTakeHandler.rejectTakeBuffer(this);
		}

		// 1. check next slot flag is CAN_TAKE_FLAG
		int nextCursorIndex = calSlotIndex(nextCursor);
		Assert.isTrue(flags[nextCursorIndex].get() == CAN_TAKE_FLAG, "Curosr not in can take status");

		// 2. get UID from next slot
		// 3. set next slot flag as CAN_PUT_FLAG.
		long uid = slots[nextCursorIndex];
		flags[nextCursorIndex].set(CAN_PUT_FLAG);

		// Note that: Step 2,3 can not swap. If we set flag before get value of slot, the producer may overwrite the
		// slot with a new UID, and this may cause the consumer take the UID twice after walk a round the ring
		return uid;
	}

	/**
	 * Calculate slot index with the slot sequence (sequence % bufferSize)
	 */
	protected int calSlotIndex(long sequence) {
		return (int) (sequence & indexMask);
	}

	/**
	 * Discard policy for {@link RejectedPutBufferHandler}, we just do logging
	 */
	protected void discardPutBuffer(RingBuffer ringBuffer, long uid) {
		LOGGER.warn("Rejected putting buffer for uid:{}. {}", uid, ringBuffer);
	}

	/**
	 * Policy for {@link RejectedTakeBufferHandler}, throws {@link RuntimeException} after logging
	 */
	protected void exceptionRejectedTakeBuffer(RingBuffer ringBuffer) {
		LOGGER.warn("Rejected take buffer. {}", ringBuffer);
		throw new RuntimeException("Rejected take buffer. " + ringBuffer);
	}

	/**
	 * Initialize flags as CAN_PUT_FLAG
	 */
	private PaddedAtomicLong[] initFlags(int bufferSize) {
		PaddedAtomicLong[] flags = new PaddedAtomicLong[bufferSize];
		for (int i = 0; i < bufferSize; i++) {
			flags[i] = new PaddedAtomicLong(CAN_PUT_FLAG);
		}

		return flags;
	}

	/**
	 * Getters
	 */
	public long getTail() {
		return tail.get();
	}

	public long getCursor() {
		return cursor.get();
	}

	public int getBufferSize() {
		return bufferSize;
	}

	/**
	 * Setters
	 */
	public void setBufferPaddingExecutor(BufferPaddingExecutor bufferPaddingExecutor) {
		this.bufferPaddingExecutor = bufferPaddingExecutor;
	}

	public void setRejectedPutHandler(RejectedPutBufferHandler rejectedPutHandler) {
		this.rejectedPutHandler = rejectedPutHandler;
	}

	public void setRejectedTakeHandler(RejectedTakeBufferHandler rejectedTakeHandler) {
		this.rejectedTakeHandler = rejectedTakeHandler;
	}

	@Override
	public String toString() {
		StringBuilder builder = new StringBuilder();
		builder.append("RingBuffer [bufferSize=").append(bufferSize)
			.append(", tail=").append(tail)
			.append(", cursor=").append(cursor)
			.append(", paddingThreshold=").append(paddingThreshold).append("]");

		return builder.toString();
	}

}
